It is sometimes desirable to create markings on documents that cannot be seen by a human user but that may be read by a machine. Lasers are used to write and read microscopic markings on optical data storage disks, commonly known as CDs and DVDs for example. In some CD writers, a layer of laser absorbing dye on the CD is exposed to a controlled pulsing laser light to write data on to the CD. The heat generated when the dye absorbs the laser light darkens the dye at each of the locations exposed to the laser light, changing the reflectivity of the CD at those locations. A reading laser in a CD player detects the pattern of changing reflectivity to “read” the data “written” on the CD. It would be advantageous, therefore, to be able to “print” microscopic markings on paper documents and read such markings from those documents in much the same way that microscopic data is written to and read from a CD.